These days HTPC's are more about vidoe card hardware acceleration capabilities than cpu speed. Look at the popularity of the Nvidia Ion platform which mates an Atom processor to a very capable GPU. The Atom is no thoroughbred by any means but these system can play 1080p without a hitch.

The only time that cpu power comes into play is with HD non supported video formats which gpu's hardware acceleration has no support for. This is becoming the exception & not the rule.